The tense that you've going for "she would have done it" would translate in french to: "elle l'aurait fait"
Therefore I'd take what you've suggested and modify it to:
Sinon, pas question qu'elle l'aurait fait!
That being said, in written French what you've said doesn't make a lot of sense... it would therefore be rather informal. I'd change it to be more correct/formal to:
Sinon, il n'y a pas de question qu'elle l'aurait fait!
But this expression <<il n'y a pas de question que>> takes a subjonctif, right?
In that case if i used subjonctif present-
<<il n'y a pas de question qu'elle le fasse >>, would this convey what i want to say?
I'm not certain that that actually requires the subjunctive. That being said, until a native speaker has the opportunity to chime in here's what it would be if you needed to use the subjunctive:
Il n'y a pas de question qu'elle l'eût fait
PLEASE be aware that I may be totally misrepresenting what it should be since, as confident as I am in my French, when it comes to these very bizarre tenses I have to differ to native speakers.
Isn't it so that «pas question» means "no way!", as in, "There's no way that she would have done it"? «Il n'y pas de question» suggests, using the example above, that "there is no question that she would have done it".
"otherwise there's no question that she would have done it."
We would need more context for the sentence in English.
I guess it might also mean: "... there is no way she would have done it":
"il n'était pas question/il était hors de question qu'elle ait pu le faire/ qu'elle l'ait fait"
But "...that she would" makes me think that my first attemps are closer to what was asked in the first post.